In U.S. Pat. No. 3,403,207 (S.I. Kreps et.al.) are disclosed sun screen compositions based on derivatives of para-aminobenzoic esters. The common preparation of these esters is carried out by esterification of the corresponding acid using the appropriate alcohol. The particular acid can be obtained by reductive methylation of p-nitrobenzoic acid in an ethanolic solution. Another process for directly obtaining the esters comprises the N-methylation of p-aminobenzoic acid using a system based on Me.sub.2 SO.sub.4 - KOH (cf chemical Abstracts 77, 5110K).
An interesting process is described in J.Org.Chem. 44 (13), 2199, 1979, in which the carbonylation of p-Br-dimethylaniline is carried out with CO in the presence of a catalyst based on NaH-RONaCo(OAc).sub.2. The process has the disadvantage that products with very low carbonylation yields (35-40%) and high content of impurities are obtained.
In U.S. Pat. No. 2,640,071 a method is described for the preparation of carboxylic acid derivatives based on the carbonylation of aromatic halides. The reaction is catalyzed by compounds of metals from group VIII at temperatures in the range of 250 to 450.degree. C. and carbon monoxide pressures in the range of 300 to 1000 atmospheres.
In a very recent U.S. Pat. No. 4,654,436, aromatic carboxylic acid esters are obtained by reacting an aromatic halide, such as benzene halide, naphthalene halide, with carbon monoxide in the presence of a palladium catalyst promoted with metal carbonyls, said metal being selected from group VIB. As mentioned in the specification, the most preferred reaction temperatures are in the range of between 100.degree. to 125.degree. C. From the Examples given, conversions in the range of 67% to 95% are claimed to be obtained, but no data are specified concerning the impurities present in the products. However, it is stipulated and examplified in the specification, that in the absence of a promoter very poor conversions in the order of 16% to 20% are obtained.
It should be clearly understood that for human skin protection, the problem of impurities present in the reagents used therein is very critical.
It is an object of the present invention to provide a simple method for the manufacture of dimethyl-octyl (or pentyl) para- aminobenzoic ester. It is another object of the present invention to provide a simple method for the manufacture of the above compounds at very high conversions. It is yet another object of the present invention to provide a simple method for the manufacture of the above compounds in a substantially pure form.